1. Field of the Invention
The present invention relates to a cobalt-based magnetic thin film material which can be used for high-density magnetic recording devices. More particularly, the present invention relates to a thin film material consisting of certain Co-based alloys containing phosphorous, such as Co-Cr-P-Pt or Co-Cr-P-Ni alloy, which can provide a high coercive force and a high squareness ratio.
2. Description of the Prior Art
In recent years, much efforts have been made for achieving higher recording density in the fields of magnetic recording and reproduction devices such as computer hard disk drives and VTRs. This has resulted in a demand for recording media having a higher coercive force and appearing in the form of an ultra-thin film. Attempts have also been made to improve the signal to noise ratio.
Conventionally, cobalt-based alloys have mainly been used as thin film materials for magnetic recording devices. Co-Cr-Ta alloys are most frequently employed as magnetic thin film materials for hard disks.
To enhance the coercive force of thin films formed from these materials, a substrate is usually heated to a temperature above 200.degree. C. before sputtering or vapor deposition is initiated to form a thin film on it. Also, bias is often applied to the substrate during sputtering or vapor deposition. However, the coercive force of a thin film formed from the above materials is about 1600 Oersted (Oe) for heated substrates, and at most 1800 Oe for heated substrate to which biasing has been applied.
The above methods for enhancing coercive force of magnetic thin films have some problems. In the substrate heating method, elevation of the substrate temperature is limited due to the risk that the substrate will be distorted or the noncrystalline Ni-P layer will be crystallized by the heating in the case of Al substrates. In the substrate bias method, the substrate is restricted since it must be an electrically conductive material. Glass and ceramics are considered the most preferable substrate material for hard disks for high density magnetic recording devices. Both are electrically nonconductive materials, and thus the substrate bias method is not suitable for preparing a magnetic thin film having a high coercive force using a substrate comprised of these materials.
The recent trend is toward reducing the distance between the head and the recording medium of a magnetic recording device in order to compensate for the lowering of reproduced signals expected from the reduction of residual magnetic flux x magnetic layer thickness (M.sub.r .multidot.t) for increasing recording density. Flatness of the substrate is very important in this context. Therefore, it is expected that substrates comprised of glass or ceramics will be incorporated since they are superior to those comprised of Al alloys in maintaining flatness. Furthermore, Al alloy disks presently have been getting thinner in general, so that the possibility of distortion by heating is increased.
Co-Cr-Pt and Co-Cr-Pt-Ta are the magnetic thin film materials which can provide coercive forces above 1800 Oe. However, these materials have the disadvantages that they contain a substantial amount of an expensive metal, Pt, and their large crystallite size and poor separation between crystals causes high noise when they are formed into a thin film for magnetic recording devices.
Japanese Ko-Kai (Hei) 4-221418 and IEEE Trans. Magn., Vol. 28, No. 5, p 3084 (1992) disclose a magnetic thin film comprised of Co-Cr-Pt-B. This film has problems that high coercive force is obtained only when the substrate is heated to a high temperature and bias is applied to it during the formation of the thin film. Thus, the manufacturing process is complex and a high coercive force cannot be obtained when substrates made of glass or the like are used.